Conventionally, in recognizing or measuring a pattern using electron beams of an SEM, a low acceleration voltage (1 kV or lower ) and low current (10 pA or less) have been used for the SEM in order to reduce radiation damages of a subject or prevent charge-up. Image processing such as a frame accumulation process or filtering process has also been carried out in order to improve an S/N ratio of an image obtained from secondary electron beams of an SEM. A pattern of a processed image obtained by secondary electron beams is automatically measured in accordance with various types of algorithms heretofore proposed (e.g., a threshold value method, a linear approximation method, and a maximum slope method).
The surface of a subject underwent a trench forming process among VLSI manufacturing processes has generally a flat configuration (about 0.1 .mu.m irregularity). In addition, insulation material such as photoresist covers the subject surface. Accordingly, the contrast of an SEM image obtained for searching a pattern is very poor to the extent that a desired pattern location can scarcely be detected. After a desired pattern is once detected, the magnification factor of the SEM is made increased to observe the pattern more finely. With this operation, the dose increases. For this reason, it often becomes impossible to measure a pattern because of increased charge-up while SEM or EOS parameters are adjusted. It is very important to correctly extract pattern features.
Apart from the above, a frame accumulation process is performed in order to improve an S/N ratio of an image to be observed. In this case, an area radiated by electron beams is likely to be contaminated. In view of this, the number of frame accumulations has been limited based on the degree of contamination and charge-up. As a result, not a few cases have occurred wherein an obtained image does not allow automatic measurement of the bottom area, diameter, or the like of a contact hole pattern, for example.